feat: MSIX packaging for Store distribution and code signing

[DarthVaderMS]

Summary

Package the app as MSIX for Microsoft Store distribution and/or signed sideloading. This is the long-term packaging goal.

Why MSIX

• Solves SmartScreen entirely — MSIX packages are always signed; no more "Windows protected your PC" prompts
• Virtualises file writes — even if SettingsManager writes to AppContext.BaseDirectory, the OS transparently redirects writes to a per-user location (so bug: SettingsManager writes appsettings.json to AppContext.BaseDirectory instead of %APPDATA% #22 becomes optional)
• Replaces Run registry autostart with a declared StartupTask extension — cleaner, user-controllable from Task Manager's Startup tab
• Clean install/uninstall — no registry leftovers, no orphaned files
• Microsoft Store distribution — massively increases discoverability
• Auto-update via the Store or AppInstaller files

Scope

• Add installer/Package.appxmanifest with StartupTask extension (replaces WindowsStartupManager registry writes)
• Add installer/HomeAssistantWindowsVolumeSync.wapproj (Windows Application Packaging Project)
• Acquire a code signing certificate (EV cert recommended for SmartScreen reputation)
• GitHub Actions workflow to build and sign the MSIX on release
• Submit to Microsoft Store (optional)

Notes

• Requires Visual Studio or makeappx/signtool in CI
• EV code signing certificate costs ~–500/year but is the gold standard
• If Store submission is not desired, the MSIX can be distributed as a sideload with an AppInstaller file for auto-updates

Dependencies

• None hard — but bug: SettingsManager writes appsettings.json to AppContext.BaseDirectory instead of %APPDATA% #22 (config in %APPDATA%) is still good practice even with MSIX write virtualisation

Related

Part of installer track: #17